The present invention relates to a scanning electron beam apparatus for observing a large size sample with high resolution while blocking external disturbing magnetic fields.
In recent years, low voltage acceleration of electrons has been proposed for observation of a semiconductor sample with a minimum of damage and no charging. However, the conventional scanning electron microscope (SEM) normally has insufficient resolution. In view of this, there has been developed a so-called in-lens SEM in which a sample is disposed within a magnetic field of an objective lens. However, this in-lens SEM can observe only a moderate size sample up to 10 mm in diameter. Accordingly, the in-lens SEM is not suitable for observation of a large size sample such as a 6-inch wafer which finds widespread use in the semiconductor industry. Therefore, another type of the SEM has been sought for observing a large size sample with high resolution.
For this purpose, a particular SEM is disclosed in Japanese Patent Application Laid Open No. 161235/1983. As shown in FIG. 8 of the present application (and referring to FIG. 5 of the above prior art application), the SEM disclosed in the cited Japanese application is composed of an objective lens 29 which has an upper magnetic pole 20 and a lower magnetic pole 21 separated from each other. These magnetic poles define therebetween a sample accommodation space 28 extending transversely of an alignment direction of the magnetic poles, and a sample holder 14a is interposed in a gap between the magnetic poles. The sample holder 14a has an area larger than that of the objective lens magnetic pole face. In the above-noted prior art application, the lower magnetic pole may be eliminated (as shown in FIG. 10 of the prior art application) such that sample accommodation space 28 can be enlarged to arrange therein various types of detectors and to increase the setting freedom of the sample holder.
Further, there has been proposed another type of SEM for the observation of a large size sample with high resolution, in which a single-pole magnetic lens 5 is utilized as an objective lens as shown in FIG. 9A. The single-pole magnetic lens 5 has an axially symmetric structure as shown in FIG. 9A, and is composed of a central tube 10 having an end face 10a in opposed relation to a sample 13, a peripheral sleeve 8 having an edge face 9, and an exciting coil 6. The central tube end face 10a and the peripheral sleeve edge face 9 constitute a magnetic circuit of the lens, and are disposed at the same side with respect to the sample 13. Other components such as an electron gun and a condenser lens may be disposed at either of sides a and b. The electron beam may irradiate the sample 13 in either of directions 5a and 5b. In either case, the single-pole magnetic lens can realize a smaller spherical aberration coefficient Cs and a smaller chromatic aberration coefficient Cc as compared to the ordinary SEM objective lens (Disclosed Document: S. M. Juma, et al., 1983, J. Phys. E Sci. Instrum., Vol. 16 P1063).
FIG. 9B shows the magnetic flux distribution and magnetic flux density distribution along an optical axis formed by the single-pole magnetic lens. The distribution of the magnetic flux density Bz has its maximum value in the vicinity of the central tube end face 10a. The distribution decreases abruptly in direction a and gradually in the other direction b. The sample 13 can be positioned at a point where the magnetic flux density Bz has nearly its maximum value to thereby realize a high resolution as in the case of the in-lens SEM. Moreover, since the central tube end face and peripheral sleeve edge face of the single-pole magnetic lens are disposed at the same side with respect to a sample, a sufficient space can be provided for accommodating a large size sample and sample stage to thereby enable observation of a large size sample.
However, in the apparatus shown in FIG. 8, the sample holder must have a planar shape. On the other hand, a sample stage of a standard SEM must be able to undergo various movements such as XY displacement, rotation and tilting, and it would be difficult to realize these needed movements in the planar sample stage.
As described before, the prior art application (No. 161235/1985) discloses another type of conventional SEM in which a lower magnetic pole piece is removed as shown in FIG. 10 of the prior art application. In this type, a variably movable sample stage can be provided in the SEM. However, in this structure, a sample chamber constitutes a part of a magnetic circuit of the objective lens. Therefore, asymmetry of the sample chamber may affect the electron optical system, thereby causing shortcomings such as axis distortion and astigmatism.
Further, in the other prior art shown in FIG. 9A herein, the central tube end face and the peripheral sleeve edge face constitute a magnetic single-pole structure to ensure good axial symmetry. On the other hand, since a sample is not surrounded by a magnetic container, external disturbing magnetic fields cannot be blocked and can thereby cause problems especially in low voltage acceleration operation.
In response to the above problems, a magnetic shield may be provided around a sample. In this case, if the magnetic shield constitutes a part of the magnetic circuit, the magnetic field of the single-pole magnetic lens is distorted to hinder its optimum performance and to cause drawbacks such as axis dislocation and astigmatism.